Elden Ring’s combat will never be as deep mechanically as a traditional, 1v1 fighting game Guilty Gear. Even so, playing any of that series — or any fighting game title — will nevertheless help you get ready for PvP in the Lands Between.
Souls combat takes more than a few ideas from fighters, from spacing to true combo to neutral, so you best familiarize yourself with them now. The main difference is, in a Guilty Gear or Street Fighter, your character is the toolset and the only deviation is in how you play them vs. how someone else does.
In a Souls-like RPG, your weapons and abilities are your "character" and the body you control is a vector to experience the world through them.